Abstract

A continental-margin magmatic arc is inferred to have existed on the southeastern (present coordinates) margin of Laurentia from Labrador to Texas from ∼1500–1230 Ma, with part of the arc subsequently being incorporated into the 1190–990 Ma collisional Grenville Orogen. Outside the Grenville Province, where the arc is known as the Granite–Rhyolite Belt, it is undeformed, whereas within the Grenville Province it is deformed and metamorphosed. The arc comprises two igneous suites, an inboard, principally quartz monzonitic to granodioritic suite, and an outboard tonalitic to granodioritic suite. The quartz monzonite–granodiorite suite was largely derived from continental crust, whereas the tonalitic–granodiorite suite is calc-alkaline and has a juvenile isotopic signature. Available evidence from the Grenville Province suggests that the arc oscillated between extensional and compressional settings several times during the Mesoproterozoic. Back-arc deposits of several ages, that formed during relatively brief periods of extension, include (1) mafic dyke swarms subparallel to the arc; (2) continental sediments, bimodal volcanics and plateau basalts; (3) marine sediments and volcanics formed on stretched continental crust; and (4) ocean crust in a marginal basin. Closure of the back-arc basins occurred during the accretionary Pinwarian (∼1495–1445 Ma) and Elzevirian (∼1250–1190 Ma) orogenies, as well as during three pulses of crustal shortening associated with the 1190–990 Ma collisional Grenvillian Orogeny. During the Elzevirian Orogeny, closure of the Central Metasedimentary Belt marginal basin in the southeastern Grenville Province was marked by subduction-related magmatism as well as by imbrication of back-arc deposits. The presence of a continental-margin magmatic arc on southeastern Laurentia during the Mesoproterozoic implies that other coeval magmatism inboard from the arc took place in a back-arc setting. Such magmatism was widespread and chemically diverse and included large volume “anorogenic” anorthosite–mangerite–charnockite–granite (AMCG) complexes as well as small volume alkaline, quartz-saturated and -undersaturated “within-plate” granitoids. Recognition of the ∼300 million year duration of the Mesoproterozoic convergent margin of southeastern Laurentia suggests that there may be useful parallels with the evolution of the Andes, which has been a convergent margin since the early Paleozoic.

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